This invention relates to a plated steel material having enhanced corrosion resistance and workability, as required for outdoor and exposed uses such as structures, revetments, fishing nets, fences, etc., and a method to produce the plated steel material. The plated steel material includes: plated steel wires such as steel wires for gauze, concrete reinforcing fibers, bridge cables, PWS wires, PC wires, ropes and the like; structural steels such as H sections, sheet pilings and the like; machine components such as screws, bolts, springs and the like; steel sheets and plates; and other steel materials.
Among plated steel materials, and among plated steel wires in particular, galvanized steel wires and zincxe2x80x94aluminum alloy plated steel wires, superior to galvanized steel wires in corrosion resistance, are commonly used. The zincxe2x80x94aluminum alloy plated steel wires are produced, generally, by subjecting a steel wire to the following sequential processes: washing, degreasing, or other means of cleaning; flux treatment; plating by either a two-step plating process consisting of a first step of hot dip plating in a plating bath mainly containing zinc and a second step of hot dip plating in a Znxe2x80x94Al alloy bath containing 10% of Al or a one-step plating process in a Znxe2x80x94Al alloy bath containing 10% of Al; then, after the wire vertically extracted from the plating bath, cooling it and winding it into coils.
The good corrosion resistance of a zincxe2x80x94aluminum alloy plated steel wire is enhanced yet further by increasing the plating thickness. One of the methods to secure a desired plating thickness is to increase the speed of a steel wire (wire speed) at plating operation so that it comes out of a plating bath at a high speed and to increase the amount of the plated alloy adhering to the steel wire owing to the viscosity of the molten plating alloy. By this method, however, the plating thickness of a plated steel wire, in the cross section perpendicular to its longitudinal direction, is likely to become uneven because of the high speed, and therefore there is a limitation related to a plating apparatus. Consequently, galvanizing or hot dip plating of Znxe2x80x94Al alloy using current plating apparatuses cannot provide sufficient corrosion resistance and there is a problem that today""s strong demands for a longer service life of a plated steel wire are not satisfactorily fulfilled.
To cope with the problem, Japanese Unexamined Patent Publication No. H10-226865 proposes a plating composition of a Znxe2x80x94Alxe2x80x94Mg alloy system, wherein corrosion resistance is enhanced by the addition of Mg to a plating bath. However, the plating method based on this plating composition is meant for a small plating thickness on steel sheets and when the method is applied to heavy plating steel wires represented by steel wires for outdoor exposed uses such as structures, revetments, fishing nets, fences, etc., there occurs a problem that cracks develop in the plated layers during the working of the plated steel wires. Japanese Unexamined Patent Publication No. H7-207421 discloses a method to apply Znxe2x80x94Alxe2x80x94Mg alloy plating of a heavy plating thickness. When this method is applied to the plating of steel wires without modification, however, a thick Fexe2x80x94Zn alloy layer forms and there is a problem that the Fexe2x80x94Zn alloy layer cracks or peels off during the working of the plated steel wires.
The object of the present invention is, in view of the above problems, to provide a hot dip zinc alloy plated steel material, particularly a hot dip zinc alloy plated steel wire, excellent in corrosion resistance and workability which does not suffer cracks and exfoliation in a plated layer and/or a plated alloy layer during the working of the plated steel wire, and a method to produce the plated steel wire.
The present inventors established the present invention as a result of studying the means to solve the above problems and the gist of the present invention is as follows:
(1) A plated steel material excellent in corrosion resistance and workability, characterized by having an alloy layer 20 xcexcm or less in thickness consisting of, in mass, 25% or less of Fe, 30% or less of Al, 5% or less of Mg and the balance consisting of Zn, at the interface of a plated layer and a base steel.
(2) A plated steel material excellent in corrosion resistance and workability, characterized by having: an alloy layer 20 xcexcor less in thickness consisting of, in mass, 25% or less of Fe, 30% or less of Al, 5% or less of Mg and the balance consisting of Zn at the interface of a plated layer and a base steel; and the plated layer consisting of, as an average composition in mass, 4 to 20% of Al, 0.8 to 5% of Mg, 2% or less of Fe and the balance consisting of Zn, on top of the alloy layer.
(3) A plated steel material excellent in corrosion resistance and workability, characterized by having, at the interface of a plated layer and a base steel, an alloy layer composed of: an inner alloy layer 5 xcexcm or less in thickness consisting of, in mass, 15% or more of Fe, 20% or more of Al, 2% or more of Si, 5% or less of Mg and the balance consisting of Zn; and an outer alloy layer 30 xcexcor less in thickness consisting of, in mass, 25% or less of Fe, 30% or less of Al, 2% or more of Si, 5% or less of Mg and the balance consisting of Zn.
(4) A plated steel material excellent in corrosion resistance and workability, characterized by having: at the interface of a plated layer and a base steel, an alloy layer composed of an inner alloy layer 5 xcexcm or less in thickness consisting of, in mass, 15% or more of Fe, 20% or more of Al, 2% or more of Si, 5% or less of Mg and the balance consisting of Zn and an outer alloy layer 30 xcexcm or less in thickness consisting of, in mass, 25% or less of Fe, 30% or less of Al, 2% or more of Si, 5% or less of Mg and the balance consisting of Zn; and, on top of the outer alloy layer, the plated layer consisting of, as an average composition in mass, 4 to 20% of Al, 0.8 to 5% of Mg, 0.01 to 2% of Si, 2% or less of Fe and the balance consisting of Zn, and containing Mg2Si dispersively existing therein.
(5) A plated steel material excellent in corrosion resistance and workability according to the item (2), characterized in that the solidification structure of the plated layer is a granular crystal structure or a columnar crystal structure.
(6) A plated steel material excellent in corrosion resistance and workability according to the item (2) or (4), characterized in that each of an a phase mainly composed of Alxe2x80x94Zn, a xcex2 phase consisting of Zn only or an Mgxe2x80x94Zn alloy layer and a Znxe2x80x94Alxe2x80x94Mg ternary eutectic phase exist in the structure of the plated layer.
(7) A plated steel material excellent in corrosion resistance and workability according to the item (6), characterized in that the volume percentage of the xcex2 phase existing in the structure of the plated layer is 20% or less.
(8) A plated steel material excellent in corrosion resistance and workability according to the item (2) or (4), characterized in that the plated layer further contains one or more of the elements selected from among one or more of the groups of a, b, c and d below;
a: one or more elements of Ti, Li, Be, Na, K, Ca, Cu, La and Hf in 0.01 to 1.0 mass % each,
b: one or more elements of Mo, W, Nb and Ta in 0.01 to 0.2 mass % each,
c: one or more elements of Pb and Bi in 0.01 to 0.2 mass % each,
d: one or more elements of Sr, V, Cr, Mn and Sn in 0.01 to 0.5 mass % each.
(9) A plated steel material excellent in corrosion resistance and workability according to any one of the items (1) to (8), characterized in that the plated steel material further has any one of a paint coating and a heavy anticorrosion coating.
(10) A plated steel material excellent in corrosion resistance and workability according to the item (9), characterized in that the heavy anticorrosion coating consists of one or more of the high molecular compounds selected from among vinyl chloride, polyethylene, polyurethane and fluororesin.
(11) A plated steel material excellent in corrosion resistance and workability according to any one of the items (1) to (10), characterized in that the plated steel material is a plated steel wire.
(12) A method to produce a plated steel material excellent in corrosion resistance and workability, characterized by: applying to a steel material a hot dip galvanizing containing, in mass, 3% or less of Al and 0.5% or less of Mg as the first step, and then a hot dip alloy plating consisting of, as an average composition in mass, 4 to 20% of Al, 0.8 to 5% of Mg, 2% or less of Fe and the balance consisting of Zn as the second step, so as to form an alloy layer 20 xcexcm or less in thickness consisting of, in mass, 25% or less of Fe, 30% or less of Al, 5% or less of Mg and the balance consisting of Zn at the interface of a plated layer and a base steel; and then making the solidification structure of the plated layer a granular crystal structure by cooling the plated steel material at a cooling rate of 300xc2x0 C./sec. or less or a columnar crystal structure by cooling the plated steel material at a cooling rate of 300xc2x0 C./sec. or more.
(13) A method to produce a plated steel material excellent in corrosion resistance and workability, characterized by: applying to a steel material hot dip galvanizing containing, in mass, 3% or less of Al and 0.5% or less of Mg as the first step, and then a hot dip alloy plating consisting of, as an average composition in mass, 4 to 20% of Al, 0.8 to 5% of Mg, 0.01 to 2% of Si, 2% or less of Fe and the balance consisting of Zn as the second step, so as to form an alloy layer composed of an inner alloy layer 5 xcexcm or less in thickness consisting of, in mass, 15% or more of Fe, 20% or more of Al, 2% or more of Si, 5% or less of Mg and the balance consisting of Zn and an outer alloy layer 30 xcexcm or less in thickness consisting of, in mass, 25% or less of Fe, 30% or less of Al, 2% or more of Si, 5% or less of Mg and the balance consisting of Zn at the interface of a plated layer and a base steel; and then making the solidification structure of the plated layer a granular crystal structure by cooling the plated steel material at a cooling rate of 300xc2x0 C./sec. or less or a columnar crystal structure by cooling the plated steel material at a cooling rate of 300xc2x0 C./sec. or more.
(14) A method to produce a plated steel material excellent in corrosion resistance and workability according to the item (12) or (13), characterized in that the hot dip alloy plating of the second step further contains one or more of the elements selected from among one or more of the groups of a, b, c and d below;
a: one or more elements of Ti, Li, Be, Na, K, Ca, Cu, La and Hf in 0.01 to 1.0 mass % each,
b: one or more elements of Mo, W, Nb and Ta in 0.01 to 0.2 mass % each,
c: one or more elements of Pb and Bi in 0.01 to 0.2 mass % each,
d: one or more elements of Sr, V, Cr, Mn and Sn in 0.01 to 0.5 mass % each.
(15) A method to produce a plated steel material excellent in corrosion resistance and workability according to the item (12) or (13), characterized by: conducting the first step hot dip galvanizing at an immersion time of 20 sec. or less in a plating bath and then the second step hot dip zinc alloy plating at an immersion time of 20 sec. or less in another plating bath; and, at both the first and second steps of the plating, purging the areas where the steel material is pulled up out of the plating bathes with nitrogen gas in order to prevent the plating bath surface and the plated steel material from oxidizing.
(16) A method to produce a plated steel material excellent in corrosion resistance and workability according to the item (12) or (13), characterized by solidifying the plated alloy by direct cooling using any one of the cooling means of water spray, gas-atomized water spray or water flow immediately after the plated steel material is pulled up from the plating bath of the second step hot dip zinc alloy plating.
(17) A method to produce a plated steel material excellent in corrosion resistance and workability according to the item (12) or (13), characterized by commencing the cooling of the plated steel material at a temperature 20xc2x0 C. or less above the melting point of the plating alloy.
(18) A method to produce a plated steel material excellent in corrosion resistance and workability according to any one of the items (12) to (17), characterized in that the plated steel material is a plated steel wire.